VSE: Use png images in one channel as alpha mask for another channel
I agree that it is a bit hidden.

I have three strips here, a blue one which serves as the background, an orange one as foreground and the black and white at the bottom to use as alpha channel.

The orange strip on top is supposed to get the mask. In order to do that, it needs a Mask Modifier with the Black &amp; white strip as its source.

After that, simply select 'Alpha Over' as the blend type for the top strip.



The alpha mask:



What you get with the default blend type (Cross):



The finished result: